The acyclic nucleotide analogues, HPMPC and PMEA, differ in their in vitro effect on the genetic material of eukaryotic cells. While HPMPC exerts a cytostatic effect on eukaryotic cells in vitro, PMEA has a genotoxic activity. These results correspond with the mode of embryotoxic action of these compounds: HPMPC exhibits a general embryolethal effect, whereas PMEA is apparently teratogenic and interacts with the mutant allele producing preaxial polydactyly of the hind limbs.